1. Field of the Invention
The present invention relates to a fuel cell stack and a method of holding such a fuel cell stack under pressure, and more particularly to a fuel cell stack which keeps unit cells reliably in electric contact with each other even if the unit cells suffer dimensional changes due to thermal expansion or shrinkage due to temperature changes, and a method of holding such a fuel cell stack under pressure.
2. Description of the Related Art
FIG. 9 of the accompanying drawings shows in enlarged fragmentary longitudinal cross section a general fuel cell stack 10 mounted on a vehicle body 1 of a vehicle such as an automobile or the like. The fuel cell stack 10 comprises a stacked body 13 comprising a plurality of unit cells 12 electrically connected in series to each other and stacked in a horizontal direction in FIG. 9.
Each of the unit cells 12 comprises a joint body 20 made up of an anode electrode 14, a cathode electrode 16, and an electrolyte 18 interposed between the anode and cathode electrodes 14, 16, and a pair of separators 22a, 22b sandwiching the joint body 20 therebetween. The separators 22a, 22b have first gas passages 24 defined in their surfaces facing the anode electrodes 14 for supplying a fuel gas (e.g., a hydrogen-containing gas chiefly composed of hydrogen) to and discharging the fuel gas from the anode electrodes 14, and second gas passages 26 defined in their surfaces facing the cathode electrodes 16 for supplying an oxygen-containing gas (e.g., air) to and discharging the oxygen-containing gas from the cathode electrodes 16.
Current-collecting electrodes 34a, 34b are electrically connected to the unit cells 12 which are positioned on the respective opposite ends of the stacked body 13. End plates 38a, 38b are disposed outwardly of the respective current-collecting electrodes 34a, 34b with current-insulating plates 36a, 36b interposed therebetween. Backup plates 40a, 40b are disposed outwardly of the respective end plates 38a, 38b. A plurality of springs, e.g., disc springs 42, for keeping the unit cells 12 in electric contact with each other are interposed between the end plate 38a and the backup plate 40a. 
The fuel cell stack 10 also has a plurality of through holes 44 defined in its peripheral edges which extend from the backup plate 40a to the other backup plate 40b. Tie rods 46 extend respectively through the through holes 44, and nuts 48 are threaded and tightened over the tie rods 46 to fasten the backup plates 40a, 40b, thus holding the stacked body 13, the current-collecting electrodes 34a, 34b, and the end plates 38a, 38b under pressure with the disc springs 42 being compressed.
The fuel cell stack 10 is mounted on the vehicle body 1 by mount brackets 50, 52 connected respectively to the end plate 38a and the backup plate 40b. The mount bracket 52 is positioned and fixed by being coupled to the vehicle body 1 by bolts 54, and the mount bracket 50 is slidable with respect to the vehicle body 1. Specifically, as shown in FIG. 10 of the accompanying drawings, the mount bracket 50 has an arm 56 projecting laterally from a lower end thereof and having a plurality of oblong holes 60 which have respective steps 58. Bolts 62 are inserted respectively through the oblong holes 60 and have respective heads pressed against the steps 58 under suitable forces, joining the mount bracket 50 slidably to the vehicle body 50.
To the fuel cell stack 10, there are connected a fuel gas supplying and discharging mechanism, an oxygen-containing gas supplying and discharging mechanism, and a cooling water supplying and discharging mechanism (all not shown). The cooling water supplying and discharging mechanism passes cooling water through the fuel cell stack 10, and the fuel gas supplying and discharging mechanism supplies the fuel gas to the anode electrodes 14 and the oxygen-containing gas supplying and discharging mechanism supplies the oxygen-containing gas to the cathode electrodes 16 while the fuel cell stack 10 is being kept at an elevated temperature. Hydrogen contained in the fuel gas is ionized at the anode electrodes 14, generating hydrogen ions and electrons, according to the following reaction formula (A):H2→2H++2e  (A)
The hydrogen ions move through the electrolytes 18 toward the cathode electrodes 16, whereas the electrons flow through an external circuit electrically connected to the anode electrodes 14 and the cathode electrodes 1 and are used as a DC electric energy for energizing the external circuit.
Thereafter, the electrons flow to the cathode electrodes 16, and react with the hydrogen ions that have moved to the cathode electrodes 16 and oxygen contained in the oxygen-containing gas which is supplied to the cathode electrodes 16, producing water, according to the following reaction formula (B):O2+4H++4e→2H2O  (B)
While the fuel cell stack 10 is in operation, when the stacked body 13 suffers a dimensional change due to thermal expansion in its stacked direction, the disk springs 42 are compressed a distance depending on the amount of thermal expansion. When the fuel cell stack 10 stops its operation and the temperature of the fuel cell stack 10 drops, the stacked body 13 shrinks and the disc springs 42 are extended. As the disc springs 42 are compressed or extended upon thermal expansion or shrinkage of the stacked body 13, the tightening forces on the stacked body 13 remain substantially uniform. Thus, the stacked body 13 are well held under pressure, keeping the unit cells 12 in electric contact with each other.
The electrolyte 18 is expanded and contracted in the stacked direction of the stacked body 13 due to the absorption and discharge of water generated by the above electrochemical changes and the humidity of the fuel gas and the oxygen-containing gas. In addition, the electrolyte 18 has its dimensions slightly reduced due to frequent temperature changes caused when the fuel cell stack 10 is repeatedly switched into and out of operation. Seals which hold the joint bodies 20 and the separators 20a, 20b are also subject to slight dimensional reductions because of frequent temperature changes. The fuel cell stack 10 are dimensionally changed in the stacked direction when the electrolyte 18, the seals, and the separators 22a, 22b undergo the above dimensional changes.
When the disc springs 42 are contracted or extended owing to dimensional changes of the fuel cell stack 10, the mount bracket 50 slides against the vehicle body 1.
It can be understood that when the stacked body 13 is thermally expanded or shrunk, the disc springs 42 are contracted or extended to keep the unit cells 12 in electric contact with each other. Stated otherwise, the disc springs 42 function as a pressing force maintaining mechanism for keeping the stacked body 13 under a substantially constant pressure even when the stacked body 13 is dimensionally changed in the stacked direction.
For incorporating the disc springs 42 into the fuel cell stack 10, the end plate 38a and the backup plate 40a are required to hold the disc springs 42 in position.
Therefore, the disc springs 42 themselves and the backup plate 40a increase the dimension of the fuel cell stack 10 in the stacked direction, and also increase the weight of the fuel cell stack 10.
When the tie rods 46 fasten the backup plates 40a, 40b to hold the disc springs 42 in position, the tightening forces applied by the tie rods 46 need to be substantially uniform. If the tie rods 46 are tightened less tightly than other locations, then the tightening forces may be reduced when the stacked body 13 is shrunk. As a result, the unit cells 12 of the stacked body 13 may suffer an electric contact failure, resulting in an increase in the internal resistance and lowering the electric generating capability of the fuel cell stack 10.
To avoid the above shortcomings, it is necessary to increase the tightening forces applied by the tie rods 46.
The end plates 38a, 38b need to be thick enough to withstand undue flexing under the increased tightening forces applied by the tie rods 46. The thick end plates 38a, 38b, however, tend to increase the dimension of the fuel cell stack 10 in the stacked direction of the stacked body 13. The weight of the fuel cell stack 10 is also increased of necessity, requiring increased propelling power for propelling the vehicle body 1 which carries the fuel cell stack 10 thereon. This problem also occurs when fastening members such as bands or the like are used instead of the tie rods 46.
The mount bracket 50 slidably coupled to the vehicle body 1 cannot be firmly fixed in position. If both the mount brackets 50, 52 are firmly fixed in position to the vehicle body 1, then they prevent the stacked body 13 from being thermally expanded, applying large thermal stresses to the fuel cell stack 10.
The mount bracket 50 slidably coupled to the vehicle body 1 is unable to sufficiently bear loads which are applied to the fuel cell stack 10 due to vibrations and shocks caused when the vehicle body 1 is running. The other mount bracket 52 fixed to the vehicle body 1, therefore, needs to be large enough to bear loads applied to the fuel cell stack 10 due to vibrations and shocks.
The large mount bracket 52 requires a large installation space for the fuel cell stack 10 on the vehicle body 1. The large mount bracket 52 is necessarily heavy, making the fuel cell stack 10 heavy of necessity, requiring increased propelling power for propelling the vehicle body 1 which carries the fuel cell stack 10 thereon.